1. Field of the Invention
This invention relates to an instrumentation system for pressurized water reactors and, more particularly, to a hydro-ball in-core instrumentation system which is of simplified construction and reduced cost and permits a significant reduction in the required size of the containment structure for the pressure vessel, and yet which affords reduced down time and radiation exposure during refueling and/or maintenance operations and increased accuracy of the power distribution map produced by the data derived from the instrumentation system. The invention also relates to the method of operation of the instrumentation system and the simplified and improved method of performing refueling and/or maintenance operations on the pressure vessel as afforded by the instrumentation system.
2. State of the Prior Art
In-core instrumentation systems are employed in pressurized water reactors to verify that the power distributions within the core are within predetermined, acceptable limits for operation. Several such systems have been developed to date; while functioning adequately for this purpose, existing such systems present a number of problems and disadvantages.
A typical automatic flux mapping system consists of a control counsel and a detector drive system, the latter comprising plural drive units, each of which has a movable detector connected to a flexible cable. Associated with each drive unit are rotary transfer mechanisms and a number of thimbles, or hollow tubes, which protrude into the reactor core. The rotary transfer mechanisms function as mechanical multiplexers and make it possible to probe any of the core paths of the reactor core with any of the detectors. One such system is disclosed in U.S. Pat. No. 3,858,191 entitled "Digital Multiplexed Position Indication and Transmission System", issued Dec. 31, 1974 and assigned to the assignee of the present invention; the '191 patent is incorporated herein by reference. U.S. Pat. No. 3,932,211, issued Jan. 13, 1976, entitled "Method of Automatically Monitoring the Power Distribution of a Nuclear Reactor Employing Movable In-Core Detectors" and assigned to the assignee of the present invention, also is incorporated herein by reference. As described in the '211 patent, the detectors are inserted into the reactor core during normal power operation according to a predetermined, intermittent, time program Upon insertion, the detectors are automatically driven through the core region along fixed, predetermined paths. The outputs of the detectors are recorded as a function of core location to provide a representation of the reactor power distribution. Other related patents assigned to the common assignee herewith disclosing various aspects of automatic flux mapping systems, include U.S. Pat. Nos. 4,255,324, 4,268,354 and 4,239,595, likewise incorporated herein. Ser. No. 950,651, cited in the U.S. Pat. No. 4,239,595, issued as U.S Pat. No. 4,268,354, also is incorporated herein.
The '595 patent, for example, discloses a movable in-core instrumentation system which is inserted from the bottom of the reactor vessel 8. FIG. 1 shows a basic system as disclosed in the '595 patent which is utilized for insertion of the movable miniature detectors, i.e., movable in-core neutron detectors 12, one of which is shown in an enlarged view in FIG. 1. Retractable thimbles 10, into which the miniature detectors 12 are driven, take the route approximately as shown; more specifically, the thimbles 10 are inserted into the reactor core 14 through conduits which extend from the bottom enclosure, or head, 9 of the reactor vessel 8 through the concrete shield 18 and then up to a thimble seal table 20. Since the movable detector thimbles 10 are closed at the leading (reactor) end, they are dry inside. The thimbles 10 thus serve as a pressure barrier between the reactor water pressure, i.e., 2500 psia, and the atmosphere. Mechanical seals between the retractable thimbles 10 and the conduits are provided at the seal tables 20. The conduits 22 are essentially extensions of the reactor vessel 8 with the thimbles 10 allowing the insertion of the in-core instrumentation movable miniature detectors 12. During operation, the thimbles 10 are stationary; they are retracted only under depressurized conditions such as occur during refueling or maintenance operations. Withdrawal of the thimbles 10 to the bottom of the reactor vessel 8 is also possible if work is required on the vessel internals.
The drive system for the insertion of each miniature detector includes basically a drive unit 24, limit switch assemblies 26, a five-path rotary transfer mechanism 28, a ten-path rotary transfer mechanism 30, and isolation valves 32, as illustrated in FIG. 1. Each drive unit 24 pushes a hollow helical wrap drive cable into the core 14 with a miniature detector 12 attached to the leading end of the cable and a small diameter coaxial cable, which communicates the detector output, threaded through the hollow center back to the trailing end of the drive cable. As a set of detectors 12 enters the core, output electronics are initiated and continue monitoring the detectors' performance through the entire flux scan of that set. The function of the automatic flux mapping system console SC is to automatically probe all of the required core paths, record the measurements, or readings, from the detectors 12, and present this information to the system operator and plant computer.
Principle disadvantages with this type of system are that: (a) the relatively limited flexibility of the drive cables for the detectors mandates that a relatively large bend radius be afforded in the path followed by the detector(s) and associated drive cable(s), extending from the generally vertical axial path within the vessel 8 as to a generally horizontal orientation within the conduit 22; this large bend radius requirement consumes 10 to 12 feet of extra height, beneath the bottom head 9 of the vessel 8, of the reactor containment building, and thus imposes an economic penalty on the unit; (b) the nature of the movable in-core detector requires that it employ high pressure, leaf-free thimbles and seals, increasing the potential of their becoming sources of leaks, radiation exposure, and maintenance downtime; (c) the presence of bottom penetrations in the reactor vessel increases the potential for adverse consequences in the event of a melt down accident or more severe recovery problems if a "bottom penetration" should fail and cause a LOCA (loss of cooling accident); and (d) as later clarified, since the thimbles are disposed within the core, as are the associated detectors and drive cables, retraction of both the cables and the thimbles as is required during refueling and related maintenance operations results in potential exposure of personnel to radioactive radiation, both as an inherent characteristic of the movable elements themselves and also due to the increases potential of leakage of primary fluid through seals necessitated by the requirement for movement of both the thimbles and the drive cables.
An alternative type system for determining neutron flux distribution is shown in U.S. Pat No. 3,711,714 and is known in the trade as a type of "Aero-ball" system. An "Aero-ball" system employs small diameter balls which are blown by a gas stream into sealed thimbles defining fixed guide paths within the core; after exposure, the balls are extracted from the core by reversing the gas stream. They are then read out (counted) outside of the reactor vessel to provide axial and radial power distribution data. In this system, the balls and the associated thimbles enter the reactor vessel through the top head: as a consequence, the structure of the upper internals is structurally very complicated, to permit the insertion of the high pressure thimbles through the head and the upper internals of the vessel, down into the core which is disposed in the lower internals of the vessel This complex system inflicts extra downtime and exposure to radiation during refueling operations, to accomplish the necessary removal and storage of the plural, separate internal thimble assemblies. Moreover, each of the thimbles is a long (approximately 21/2 core lengths) and very fragile structure.
Whereas these prior art systems, as noted at the outset, are functional for the intended purpose, they introduce certain problems and disadvantages as have been discussed briefly in the foregoing. As a result, there exists a real and continuing need for an improved in-core instrumentation system. More specifically, an ideal in-core instrumentation system is one which: (a) will take and readout a core power distribution map quickly and accurately; (b) does not increase the height of the containment and shield structures and, hence, the reactor building; (c) does not inflict any additional outage penalties during refueling operations; (d) does not increase operational risks; (e) is reliable, easy to operate and maintain; and (f) minimizes radiation exposure to personnel.